1. Field of the Invention
The present invention relates to humidity indicators used for indicating the relative humidity in an environment, and more particularly, to copper humidity indicators which change color at different relative humidities in an environment.
2. Description of Related Art
A variety of humidity indicators exist today. One type of humidity indicator is a humidity indicating card, which is used for many purposes including, determining the relative humidity in shipping containers and packages, such as packaged electronics and telecommunication products. The humidity indicator cards are typically impregnated with a humidity sensitive composition and dried. Most humidity indicating cards made today use cobalt chloride as the indicator, which changes color based on the level of hydration of the cobalt chloride. Cobalt chloride can change from a blue color, when it has one water of hydration, to a pink color when it has six waters of hydration. That is, the blue color becomes increasingly red with each additional water of hydration. Therefore, one can determine the approximate humidity based on the color present on the card. An array of humidity indication areas can be included on the card, wherein increasing humidity levels are required to change different areas of indication on the card from blue to pink.
A problem with humidity indicators using cobalt chloride is that the cobalt chloride is a known irritant and a toxin. Further, the use of cobalt chloride is subject to regulation when used with food and pharmaceutical products. Thus, cobalt chloride humidity indicators have limited applications and are not useful in the food and pharmaceutical industries.
Some types of humidity indicator cards have impregnated halides or other salts capable of releasing Cu2+ ions. International Publication No. WO 02/44712 discloses a humidity indicator paper impregnated with copper salts, a synergistic salt, and a dye.
Other types of humidity indicators include the use of humidity indicating gels. In US Patent Application Publication No. 2004/0051081 Moreton discloses a silica gel material impregnated with copper and bromide to provide a relative humidity indicator. U.S. Pat. No. 6,655,315 Gattiglia discloses a silica gel impregnated with copper chloride and hygroscopic salts, wherein the silica gel changes color to indicate the presence of moisture.
There remains a need for an improved humidity indicator that provides easily detectible color changes and that is non-toxic.